The traditional glass multifocal lens consists of a major element, usually composed of a crown glass, and a small button or segment, usually composed of a barium or flint glass and fused in the surface of the major. The major element provides far vision correction, while the smaller segment member provides near vision, or reading, correction. Both the structure and the production of this type multifocal lens are described in greater detail in our commonly assigned copending application Ser. No. 116,152 filed Nov. 3, 1987 in the name of M. L. Buckley, and reference is made thereto.
An alternative form of multifocal lens is known as the progressive lens. In this type of lens, the effective convex radii of a meniscus glass blank is progressively decreased by sagging the blank over a support. The curvature of the substrate is thereby imparted through the thickness to the top surface to effect a progressively changing curvature of increasing optical power or decreased focal lengths. This avoids the fusing step of the traditional procedure, but still requires a supported heat treatment for sagging.
Thus, it is necessary to support a blank or lens assembly on a substrate during either a fusion or a sagging heat treatment. In general, such support member should resist distortion at temperatures up to at least 750.degree. C., must take and maintain an optically true surface, be resistant to abrasion, have a low coefficient of thermal expansion and not adhere to the glass being heat treated.
Supports employed heretofore have been primarily bonded ceramics, that is, fired bodies composed of ceramic mixtures. They have included silicon carbide composites, as well as fired mixtures of alumina and chrome oxide, of asbestos and iron oxide, and of chromite and iron oxide. These bodies vary from a very hard, high density body that acts as a heat sink to a very soft, low density body that is insulating in nature.
The latter type tends to wear quickly so that its original, true curve surface may be lost during a heat cycle by glass abrasion. This condition is aggravated where a differential in thermal expansion rate exists between the substrate and the glass being supported. A hard, dense body obviates this problem, but requires excessive time and energy in the heating and cooling cycle to avoid glass breakage due to tensile stresses. Glass sticking has not been a serious problem with standard crown glasses, but has been encountered with the more recently introduced photochromic glasses. Finally, problems of availability and health risk have precluded various materials heretofore used, such as chrome ores and asbestos.